1. Field of the Invention
The present invention relates to electronically controlled mechanical timepieces that accurately drive pointers fixed to a wheel train bridge by using a generator to convert mechanical energy in the unwinding mode of a spring into electrical energy, and controlling the rotation cycle of the generator by driving a rotation control circuit with the electrical energy, and to control methods therefor.
2. Description of the Related Art
An electronically controlled mechanical timepiece described in Japanese Examined Patent Publication No. 7-119812 is known as one for indicating an accurate time that accurately drives pointers fixed to a wheel train bridge by using a generator to convert mechanical energy in the unwinding mode of a spring into electrical energy, and controlling a current flowing in the coil of the generator by driving a rotation control circuit with the electrical energy.
In the electronically controlled mechanical timepiece, by inputting a signal based on the rotation of the generator into a counter while inputting a signal from a quartz oscillator into the counter, comparing values in the counter, and controlling the generator based on the difference, rotation velocity is controlled. The counter compares the phase differences of reference clock pulses (Ref-pulses) and generator-rotation-cycle pulses (G-pulses), and increases the value of a U/D counter if the G-pulses are ahead, or decreases the value if the G-pulses are behind. The counter consists of a so-called integral counter.
When a value obtained by measuring the time of one cycle of the Ref-pulses is equal to a value obtained by the integral counter, braking of the generator is performed, and braking is continuously performed until measurement of the time of one cycle of the Ref-pulses terminates. Accordingly, the value of the integral counter sets a braking release time. That is, the value of the integral counter is multiplied by braking release time N at which the average velocity of the G-pulses is equal to a target velocity (Ref-pulses). In other words, integral control is employed in this system.
According to the integral control, the average velocity of a rotor over a sufficient duration can be controlled to a velocity in a set time, whereby pointers can be accurately moved at a controlled velocity because signals output in each cycle are compared, while the signals are being counted. However, the integral control has a problem in that the rotation velocity of the rotor cannot be instantly adjusted, which causes slow responsiveness. The integral control also has a problem in that a plurality of phase excursions is generated until the relationship between spring force and braking force is set so as to correspond to a target frequency.
The integral control can be expressed in the block diagram in FIG. 20.
In general, it is known that a transfer function used for a generator or motor is 1/s(sT+1). This consists of a first-order-lag transfer function 101 and an integral term 102 of 1/s. Accordingly, an integral factor is included in the generator as an object to be controlled. Bode diagrams on the assumption that only the integral control is performed for the object are shown in FIGS. 21 and 22.
In the Bode diagrams, it is required as a condition for stable rotation control that a phase allowance, i.e., the phase at a gain of zero db (gain intersection), be ahead of -180.degree. and that gain allowance, i.e., the gain at a phase of -180.degree. (phase intersection), be not more than zero db.
However, in the case where only the integral control is performed, a phase delay of -90.degree. occurs in the object, and a further phase delay of -90.degree. occurs due to the integral control, as shown in FIG. 21, so that the phase is at approximately -180.degree.. Thus, stable control is difficult because the integral control alone cannot obtain phase allowance and gain allowance. Accordingly, the timepiece in Japanese Examined Patent Publication No. 7-119812 must perform control at a very low frequency, and its responsive characteristic is positioned at approximately 0.016 Hz or less.
A case where the gain of the integral counter is set to be 100 times greater is shown in FIG. 22. Also, in this case, phase allowance is behind -180.degree., and stable control cannot be anticipated.
As is clear from the above-described data, by performing control using only the conventional integral control, average velocity control can be performed, but a problem occurs in that phase excursions cannot be eliminated.
In addition, slow control response causes a problem in that almost nothing can compensate for a rapid disturbance as in the case when acceleration is generated in a watch by a swing of an arm.